In spite of efforts by public health officials to encourage physical activity and reduce energy intake, the obesity rate in the U.S. and worldwide has continued to climb and it has reached epidemic proportions. According to estimates by the Center for Disease Control and Prevention in the year 2000, 30% of Americans are obese and 65% are overweight (1). One of the manifestations associated with this obesity epidemic is the growing number of people diagnosed with Non-Insulin Dependent Diabetes Mellitus (NIDDM). NIDDM is a widespread and debilitating disease characterized by insulin resistance and inflammation that can lead to coronary heart disease, hypertension, blindness, neuropathy, nephropathy, and limb amputations (2). It was recently estimated that 20.8 million Americans had NIDDM and 40.1% of middle-aged adults had prediabetes, a condition characterized by either impaired glucose tolerance or high blood fasting glucose concentrations (2). Future predictions indicate that 1 of 3 children born in 2000 will one day become diabetic (3). The impending consequence is that millions of people, if not already, will soon become dependent on oral antidiabetic medications to maintain their quality of life.
One of the most effective of the currently available medications is the thiazolidinedione (TZD) class of insulin-sensitizing drugs. Subsequent to their use as oral antidiabetic agents it was discovered that TZDs function by binding to peroxisome proliferator-activated receptor gamma (PPAR γ) (4), a nuclear receptor expressed highly in immune cells, intestine, and adipose tissue (5). The nuclear receptor superfamily, which includes the vitamin D receptor (VDR), retinoid X receptor (RXR), PPAR α, and PPAR δ, consists of 48 ligand-induced transcription factors that respond to steroid and thyroid hormones, vitamins, lipid metabolites, and xenobiotics (6, 7). After binding of a synthetic or natural agonist, PPAR γ forms a heterodimer with RXR and undergoes a conformational change that allows it to recruit coactivators (8). These coactivators, which include members of the steroid receptor coactivator (SRC) family, assist the PPAR γ RXR complex in binding to specific PPAR response elements (PPREs) in the promoter regions by increasing histone acetylation, thereby altering chromatin structure and making it more accessible (8). A primary outcome of the PPAR-controlled transcriptional regulation of genes is a reduction in the hyperlipidemia, hyperglycemia, and hyperinsulinemia associated with insulin resistance (9), though the extent to which each tissue contributes to this response is still unclear.
While PPAR γ is expressed in a number of different organs that contribute to glucose homeostasis, including skeletal muscle, pancreas, and liver, white adipose tissue (WAT) is believed to represent the primary site of TZD action (10μ, 11). Adipose tissue is an extremely bioactive organ that produces a number of hormone-like polypeptides called adipokines, which regulate a wide-range of metabolic, immune and inflammatory processes throughout the body (12). Problems arise, however, when adipocytes become hypertrophic and dysfunctional during the onset of obesity. Obesity promotes the secretion of pro-inflammatory adipokines, such as leptin, plasminogen activator inhibitor 1 (PAI-1), tumor necrosis factor alpha (TNF-α) and interleukin 6 (IL-6), and a suppression in the secretion of adiponectin, an anti-inflammatory and glucose-sensitizing polypeptide (13, 14). The pro-inflammatory adipokines can disrupt insulin signaling by promoting serine phosphorylation of insulin receptor substrate 1 (IRS-1) (15). Obesity is also associated with the infiltration of bone-marrow derived macrophages, which become the key producers of pro-inflammatory mediators in WAT (16). PPAR γ activation by synthetic agonists such as rosiglitazone reduces macrophage infiltration in WAT, a sign of WAT inflammation, and increases the number of smaller, more insulin-sensitive adipocytes in the subcutaneous region (17, 18). Adipokine production is also modulated to favor the production of adiponectin and inhibit the secretion of pro-inflammatory compounds (19).
The side effects of TZDs such as weight gain, hepatotoxicity and congestive heart failure have limited their use by millions of diabetic patients (9, 20). For instance, troglitazone (REZULIN) was launched in 1997 and withdrawn from the market in March of 2000 due to reports of serious liver injury when compared to other TZDs (21), while other Food and Drug Administration (FDA)-approved TZDs for NIDDM treatment, including rosiglitazone (AVANDIA) and pioglitazone (ACTOS), continue to be widely prescribed, concerns regarding their safety persist. In this regard, the FDA recommended that the presence of liver enzymes in blood of diabetic patients taking AVANDIA be periodically monitored. Furthermore, due to the risk factors and side effects connected with TZDs and other oral antidiabetic agents, there are no preventative medications currently available for the millions of people with prediabetes.
Abscisic acid (ABA) increases PPAR γ activity in vitro (5), improves glucose tolerance and ameliorates adipose tissue inflammation in the db/db mouse, a transgenic model of obesity, and in wild-type C57BL6/J mice fed high-fat diets (5, 6). Improvements in glucose tolerance by ABA were shown to be dependent on PPAR γ being expressed in hematopoietic cells (6). The inventors have also previously demonstrated that ABA reduces immune cell infiltration and inflammation in aortas of ApoE-deficient mice (7). In this study the mechanism of ABA was more consistent with an upregulation of the intracellular messenger cAMP (7), and other recent studies have shown that cAMP upregulation is involved in ABA's mechanism of action in mammalian cells (8, 9). In all studies conducted with ABA, there have been no side-effects observed.
While the role of ABA as a phytohormone has been studied extensively, there have been no studies that explore the effect of ABA as a dietary supplement or in treatment of diabetes or inflammation.
U.S. Pat. No. 3,958,025 to Livingston teaches a method of treating a vitamin deficiency of abscisic acid in man, animal, or avian species. The patent does not address the fact that abscisic acid is not considered by those of skill in the art as an essential vitamin, nor its requirement in the diet. In addition, the method disclosed in U.S. Pat. No. 3,958,025 does not teach the use of abscisic acid to treat or prevent diabetes and inflammation, including obesity-related inflammation.
Inflammatory bowel disease (IBD), with its two clinical manifestations Crohn's Disease (CD) and Ulcerative Colitis (UC), is characterized by significant inflammation and immune cell infiltration into the gastrointestinal (GI) tract, resulting in pain and discomfort for those afflicted (66). IBD is often managed by an array of treatments, which traditionally include aminosalicylates, corticosteroids, immunosuppressors, and anti-tumor necrosis factor α (TNF-α) therapy. The type of treatment used depends on the clinical goal (induction or remission-maintenance), the severity of inflammation, the response to current or prior treatment, and the presence of complications (67). Current IBD treatments can carry with them significant side-effects, such as weight gain, cushingoid appearance, osteopenia/osteoporosis, pancreatitis, and systemic immune suppression (68), and there is an overall need for more safe and effective treatments.
Recent clinical trials have shown that synthetic agonists of the nuclear receptor PPAR γ effectively ameliorate IBD. The PPAR γ activating anti-diabetes drugs AVANDIA and ACTOS have been shown to significantly attenuate the inflammatory response of immune cells (69). These synthetic compounds, however, have been associated with cardiovascular complications, and therefore their use for IBD has been contraindicated.
Based on findings that ABA reduces inflammation and has been shown to act through a PPAR γ-dependent mechanism, the inventors wished to assess the efficacy of ABA as a prophylactic or therapeutic option for the treatment of IBD. It was found that ABA significantly ameliorated dextran sodium sulfate (DSS)-induced colitis and reduced colonic inflammation. Two noteworthy observations arose from those findings. First, ABA significantly decreased adhesion cell marker expression in the colon. Second, mice supplemented with ABA showed significantly elevated CTLA-4 expression on CD4+ T cells in whole blood. CTLA-4 is an immunosuppressive protein whose polymorphisms have not only been linked to IBD, but also autoimmune disorders and transplant outcomes (75). Thus, evidence is herein provided that ABA is a promising agent for the treatment or prevention of IBD and may be useful for disorders in which cellular adhesion molecule expression or lymphocyte hyperactivation play pathogenic roles.